This invention relates to a device and a method of manufacturing pure water used at a manufacturing process of a semiconductor device, and particularly relates to a device and a method of manufacturing pure water with reduced densities of dissolved oxygen and total organic carbon.
Recently, a property required to a ultra LSI device becomes severe accompanied by formation of micro-pattern and there rises a problem of trace level contamination to a device at a manufacturing process. In order to prevent the trace level contamination, it is necessary at first to eliminate surface contamination of a semiconductor substrate on which the device is formed. Therefore, a technique of purifying a cleaning agent for cleaning the surface of the substrate is being developed to have high purity. In a cleaning process, pure water (ultrapure water with extremely less impurity) used for eliminating the cleaning agent remaining on the surface of the substrate is also required to have high purity. It is being cleared that a performance of the semiconductor device is remarkably increased by decreasing densities of total organic carbon (herein after referred to it as TOC) and dissolved oxygen (herein after referred to it as DO) in the pure water.
As a method for decreasing the density of DO in the pure water, Japanese Patent Application Laying Open Gazette No. 2-126992 discloses, in addition to a method that oxygen is substituted with nitrogen under a reduced pressure, a membrane deaeration method in which the pure water flows in a micro pipe made of resin capable of transmitting gas and a gas dissolved in the pure water is eliminated by reducing pressure outside of the micro pipe. This method is effective for a case where nitrogen deteriorates the device characteristics.
Japanese Patent Application Laying Open Gazette No. 2-152589 proposes a method for eliminating TOC by irradiating ultraviolet-ray to pure water into which hydrogen peroxide H.sub.2 O.sub.2 is implanted (or ozone is implanted directly). In detail, oxygen atom is generated in such a manner that H.sub.2 O.sub.2 particularly absorbs ultraviolet-ray of shorter wavelength than 365 nm and O.sub.3 particularly absorbs 254 nm ultraviolet-ray, which causes generation of OH radical in the presence of water. Then, organic matter is finally resolved by the OH radical into H.sub.2 O and CO.sub.2.
The TOC eliminating method by implanting hydrogen peroxide and irradiating ultraviolet-ray, however, requires devices and processes for eliminating hydrogen peroxide remaining after implant of the hydrogen peroxide and oxidation. This involves a complicated construction of the pure water manufacturing device and increase of processes, thus raising the manufacturing cost.
For avoiding the problem accompanied by the hydrogen peroxide implant, "Chemical Engineering", Vol. 5, No. 6 (1987), pages 435-440, discloses that without the hydrogen peroxide implant, the TOC is oxidized by only irradiating short-wavelength ultraviolet-ray (ozone ray) of, for example, 184.9 nm by a low pressure mercury-vapor lamp. This method utilizes the fact that OH radical is generated by dissolving water by an energy of irradiation of the short-wavelength ultraviolet-ray, so that hydrogen peroxide implant is not required. A ultraviolet irradiating device according to the method such as NMS-type TOC-UV by NOMURA MICRO SCIENCE CORPORATION is put into practice.
When the pure water is manufactured by combination of elimination of the TOC by irradiating the short-wavelength ultraviolet-ray and elimination of DO according to the membrane deaeration method, however, following problems arise.
For example, as shown in FIG. 7, when an oxidation device 2 for oxidizing TOC by irradiating the short-wavelength ultraviolet-ray, an ion eliminating device 3 for eliminating ion caused by oxidation of TOC, a membrane deaerator 1 for eliminating DO according to the membrane deaeration method and a particle eliminating device 4 are provided in order at a passage for treating pure water, the TOC is effectively eliminated by the ion eliminating device 3. However, when DO is eliminated by the membrane deaerator 1 after the TOC is eliminated, an TOC component increases by dissolving an eluent from a resin micro pipe of the membrane deaerator 1.
On the other hand, as shown in FIG. 8, when the membrane deaerator 1, the oxidation device 2, the ion eliminating device 3 and the particle eliminating device 4 are provided in order at the passage for treating pure water, the TOC component eluted from the membrane deaerator 1 is eliminated by the oxidation device 2 and the ion eliminating device 3. As a result the TOC density might be extremely low.
However, it is confirmed that the TOC density can hardly be decreased further than a certain degree even when the short-wavelength ultraviolet-ray is irradiated by the oxidation device 2 after deaeration by the membrane deaerator 1. This seems because even with OH radical generated by irradiating the short-wavelength ultraviolet-ray, organic matter with less oxidizing efficiency exists and DO is necessary for accelerating oxidation of tile organic matter. In other words, the TOC density cannot be decreased because an oxidizer also fades when the DO density in the pure water is extremely low at the irradiation of the short-wavelength ultraviolet-ray.
According to an experiment, when the short-wavelength ultraviolet-ray is irradiated to the water-to-be-treated without deaeration, the TOC density before deaeration is not more than 1 ppb. The DO density in the pure water after treatment depends on the DO density in the water-to-be-treated and is 50-100 ppb. When the water-to-be-treated is deaerated before short-wavelength ultraviolet-ray irradiation, the TOC density after the irradiation is about 5-10 ppb. This indicates contribution of DO to oxidation of the TOC by the ultraviolet-ray irradiation.
Accordingly, with either of the methods, the DO density is decreased with no decrease Of the TOC density, and vice versa. That is, it is difficult to decrease both densities of DO and TOC.
The present invention has its object of providing a method and a device of manufacturing pure water with less impurity by decreasing the densities of DO and TOC in such a manner to providing a device for eliminating TOC by oxidizing the TOC utilizing DO without implant of hydrogen peroxide and means for controlling an amount of DO in water-to-be-treated at an upstream side to an adequate value.